An optical information recording medium having a thin transparent protective layer of which the thickness is 0.1 mm on the recording and reproducing side using a violet laser light source (of which the wavelength is approximately 400 nm) and a lens with a high NA has been proposed as a high density optical information recording medium. Such a high density optical information recording medium has a structure where a guide groove or pits for signals are formed on the surface of a thick signal substrate, on top of this, a multi-layered recording film or a metal film which is rewritable is formed, and on top of this, a transparent protective layer is formed. As for the method for fabricating a transparent protective layer of an optical information recording medium having this structure, the following two methods can be cited.
(A) A transparent film is made to adhere to the surface of a signal substrate where a multi-layered recording film or a metal film has been formed in a guide groove or pits for signals with an adhesive, so that a transparent protective layer of which the total thickness is 0.1 mm is formed of the transparent film and the adhesive (see, for example, Japanese Patent Laid-open Publication No. 2002-184037).
(B) An ultraviolet ray curable resin in liquid form is applied to the surface of a signal substrate where a multi-layered recording film or a metal film has been formed in a guide groove or pits for signals, so as to have a thickness of 0.1 mm, and after that, is cured with ultraviolet rays, so that a transparent protective layer is gained (see, for example, Japanese Patent No. 3241560 (FIG. 4)).
According to method (B) between the above described two methods, precision in the thickness greatly depends on the precision in the application of the ultraviolet ray curable resin, while according to method (A), a film having high precision in the thickness can be used for most of the portion of the transparent protective layer where high precision in the thickness of no greater than 3 μm is required, and therefore, a transparent protective layer having high precision in the thickness of no greater than 2 μm, can be gained.
It is necessary to use a transparent film having high precision in the thickness when an optical information recording medium is manufactured according to the above described method (A). A film having high precision in the thickness can generally be gained according to a manufacturing method such as an extrusion method or die casting.
An extrusion method is a method for using a die of which the slit is adjusted so that a desired film thickness can be gained, and extruding a film material, such as melt plastic, from this slit. The film material that has been extruded from the die is cooled so as to become a film. In addition, die casting is a method for forming a film by melting a film material with a solvent and dropping the melt film material from a slit in the die in the same manner as in the extrusion method, and after that, volatilizing the solvent at a high temperature.
Though it is possible to implement high precision in the thickness of the fabricated film according to any manufacturing method for a film, there is a high possibility that defects such as unevenness or scratching will occur on the surface of the film during or after fabrication. Portions of a film rub against each other, for example, when the film is rolled up after fabrication, and thus, the surface is scratched, due to dust or other particles that has entered between the portions. Such scratching or unevenness may occur on either surface of the film.
In the case where an optical information recording medium with a transparent protective layer having a thickness of 0.1 mm for recording or reproducing using a lens with a high NA is fabricated using a film having scratches or unevenness, the scratches or the unevenness are located far away from the signal recording layer on the signal substrate (a multi-layered recording film or a metal film which is rewritable and is formed on a guide groove or pits) when the scratches or the unevenness are on the outside of the transparent protective layer (on the surface of the optical information recording medium). Therefore, the beam size of recording light or reproducing light is great when signals are recorded or signals are reproduced, and thus, the scratches or the unevenness barely affect the recording quality or the reproduced signals.
In the case where the scratches or the unevenness of the film are on the surface that makes contact with the adhesive, however, the scratches or the unevenness are located near the signal recording layer on the signal substrate, making the beam size of recording light or reproducing light small, and therefore, recording light and reproducing light are easily affected by the scratches or the unevenness. A phenomenon where, for example, the amount of light reduces due to scattering, or the function of stopping the beam deteriorates due to disturbance in the wave front occurs, causing the recording quality or the reproduced signals to deteriorate.
In view of the above described problem, an object of the present invention is to provide an optical information recording medium where the recording quality and the reproducing quality are excellent, even in the case where a transparent film having a defect, such as scratching or unevenness, is used as a portion of a transparent protective layer, and the thickness of the transparent protective layer is 10 μm to 200 μm.